The use of three-prong plug electrical connectors for appliances, extension cords, electric tools, etc. has become widespread. The use of three prongs, hot, neutral and ground, makes the operation of any electric appliance, tool, etc. safer. If there is a short, the ground wire provides a safe path for current.
The standards for a three-prong plug include, hot and common or neutral prongs that are elongate and flat (usually sheet metal, backfolded at the distal end of the prong). In contrast, the ground prong is cylindrical in shape, i.e., usually a piece of sheet metal bent to form a hollow cylindrical shape or “U” shape with a rounded distal end. In a typical three-prong plug, the hot and neutral prongs are somewhat ductile being flat and may be bent from side to side when being pulled from a socket and then manually straightened again. However, the hollow cylindrical or U shape ground prong has normally been heavily worked by being bent into its hollow shape and has become work hardened or brittle, even before it is put in use. Also, it is not ductile from side to side as are the hot and common prongs. Further, the ground prong being made from sheet stock, a tongue or unbent central portion that extends from the hollow cylinder into the plug housing becomes a weak point in the ground prong design, if it is repeatedly bent (as are the hot and common prongs) when being pulled from an electrical socket.
The differences in ductility and brittleness between the hot and neutral prongs and the ground prong, results in differences in their fracture rates on the same three prong plug. In use, the brittle stiff ground prong often severely bends or fractures at its base with the rubber insulation cover of the plug. If thus deformed, an operator may then break off the ground prong. The problem with conventional three-prong plugs is that the plug will continue to work as a two-prong plug, although it will not provide the safety of a ground connection in case of a short.
Many municipal codes require or mandate discarding or non-use of electrical plugs or extension cords lacking a useable ground prong. However, in use, especially at construction sites, workmen encountering a damaged ground prong on such a connector will break off the ground prong and use the connector in an ungrounded condition, in violation of such codes.
In an effort to overcome the deficiencies of a standard three-prong plug, U.S. Pat. No. 5,622,509 provides a three-prong electrical connector having a flexible resilient grounding prong. However, the spiral wound grounding prong may be too resilient for its application as disclosed in the patent.
U.S. Pat. No. 3,914,565 discloses a three-prong plug invented in the 1970's when three-prong sockets were new in use. The solid ground prong is biased in a non-working position and must be moved into the body to close a pair of switches to provide a usable plug. The device is more complex than necessary and the added components provide more chance for failure in use. This patent depends upon the ground prong bottoming out in the socket which is not possible in all socket designs.
Other patents, U.S. Pat. Nos. 3,308,415 and 6,419,504 disclose retractable ground prongs. U.S. Pat. No. 3,205,486 discloses an extendable socket-like prong in connection with standard prongs. U.S. Pat. No. 4,318,578 discloses an electrical connector with a ground fault connector. Other electrical connections are shown at U.S. Pat. Nos. 5,772,447; 6,910,911; 1,944,296, and 1,605,904.
A need has developed for a simple three-prong electrical connector (plug) that provides the added safety feature of not functioning when the ground prong is disabled. Additionally, a need has developed for a three-prong male connector or plug that has the provision of easy repairability or replacement of the ground prong should the same become disabled.